1. Field of the Invention
The present invention relates to a method for fabricating optical elements, and in particular to a method for fabricating Fiber Bragg Grating elements and planar light circuits made thereof.
2. Description of the Related Art
In long distance fiber optic communication systems, inactive elements, such as Fiber Bragg Grating (FBG), array waveguide grating (AWG) are critical high-end elements. FBG applications, for example, include optical add-drop multiplexers (OADM), Erbium doped fiber amplifiers (EDFA) and Raman amplifiers, all of which have recently become very popular.
Fiber Bragg Grating (FBG) is commonly manufactured by subjecting optical fiber to high energy UV excimer laser. When parts of the optical fiber are subject to a high energy laser treatment, the bonding state of the inner molecular structure changes, thereby increasing the refractive index. The refractive index of the optical fiber is changed by forming grating periods using masks. When a bandwidth is an integer times a light of specific wavelength, the light will be reflected and the rest of the light passes through the optical fiber. As a result, the incoming light either passes through or is reflected by the Fiber Bragg Grating. In terms of input and output, the FBG is an optical notch filter, corresponding to a specific bandwidth (BWn), a notch frequency (fn) and a notch wavelength (λn).
The fabrication of Fiber Bragg Grating (FBG) elements is carried out by controlling the laser energy and laser exposure time, and employing masks. FIG. 1 illustrates a traditional method for fabricating a Fiber Bragg Grating element. In FIG. 1, 10 represents optical fiber, 12 is a mask, 14 is a reflective mirror, and 16 is a KrF excimer laser beam with a 248 nm wavelength. The KrF laser beam penetrates the mask 12, passes through the reflective mirror 14, and hits the optical fiber 10. The inner molecular structure of the optical fiber 10 is thus changed to form interference stripes 20 of a specific reflective index. The grating period (pitch) 22 controls the reflective index when an incoming light passes through the optical fiber. Therefore, when light 8 passes through the optical fiber 10, 24a denotes the light corresponding to the interference stripes is selected as the reflective light 24. The remaining light, denoted as 26 in the figure, is then transmitted through the optical fiber.
Due to the restrictions of the current semiconductor manufacturing process, integration of Fiber Bragg Grating elements with semiconductor wafers does not always satisfy various design requirements. The reasons for this are further discussed in the following.
Current patterns on masks are defined by electron beams with a minimum width of e-beam of 50 angstroms. As a result, the minimum pitch of patterns on masks is 50 angstroms. When patterns are transferred onto wafers by photolithography apparatus, with a magnification of 5 for example, the minimum pitch (i.e. resolution) on a wafer is 10 angstroms. In other words, if a Fiber Bragg Grating 2000 angstroms in wavelength is to be fabricated, using the masks and magnification described above, the real wavelength must be 2000+n*10 angstroms. Consequently, an FBG of 2005 angstroms in wavelength cannot be fabricated by this method. Therefore, design freedom is restricted.
Hence, there is a need for a novel method for fabricating FBG without limitations on resolutions so that various design requirements can be obtained.